1. Field of the Invention
The present invention relates to a drop-on-demand ink jet print head mounted on an ink jet printer and a method of driving the same.
2. Description of the Related Art
A conventional ink jet print head is disclosed in Japanese Patent Laid-open (Kokai) No. Sho 63-247051. FIG. 1 shows a passage (a pressure chamber) having a hard wall on one side and a shearing strain actuator wall on the other side. The shearing strain actuator wall consists of piezoelectric ceramic. An upper portion and a lower portion of these walls are joined to each other on their boundary surfaces thereof. They are also fixedly joined to a top wall and a bottom wall. Metallized metal electrodes are formed on surfaces of the both walls of the shearing strain actuator disposed on one side of the two side walls.
In the above-described ink jet printer having the shearing strain actuator on one side of the two side walls, the actuator wall is deformed toward the passage and thus pressure is provided to ink throughout the entire length of the passage, as a shearing deformation is caused in the piezoelectric ceramic by applying a pulse voltage between the electrodes. If the pressure against the ink thus applied is beyond the predetermined minimum value, the ink is jetted through a nozzle.
Also, FIG. 9(a) in the above mentioned Laid-open discloses the ink jet print head having a pair of shearing strain actuators formed on both sides of passage walls. An upper and lower portions of the passage consist of a top wall and a bottom wall. Electrodes of the actuators are formed on an inner surface of the passage and two surfaces on outside of both walls of the passage in the form of metal electrodes.
In the ink jet print head having the shearing strain actuators formed on both passage walls, when voltage is applied across the electrodes on the inner surface of the passage and the electrodes on the outside of the both walls of the passage, the equal amount of electric field is provided to the actuators on each wall of the passage reversely. Therefore, each wall facing the passage is deformed toward the passage and pressure is provided to ink in the passage thereby to jet an ink droplet through the passage.
The above mentioned Laid-open also describes that the size of an ink droplet can be controlled by changing the voltage to be applied.
However, the conventional ink jet print heads stated above have following disadvantages. There has been a large demand for graphic printing lately by requirements of users and an improvement of Host computer, etc. In order to print a graphic of a good quality, gradation is a critical factor.
Two types of the ink let print head are disclosed in the above mentioned Laid-open, i.e., the ink jet print head which jets ink by driving an actuator formed on one side of a passage (pressure chamber), and the ink jet print head which jets ink by simultaneously changing similar actuators respectively formed on both sides of a passage. In any case, pressure variation in a pressure chamber is utilized by changing the voltage to be applied in order to change the size of a droplet. Therefore, the same number of voltages to be applied as the number of gradation degrees is required. Also, an apparatus to switch a connection of a plurality of electric sources corresponding to the gradation degrees and electrodes of each actuator is required which is costly.